1. Field of the Invention
This invention is directed to a method of recovering oil from viscous oil-containing subsurface formations. More particularly, this invention is directed to an improved hydrocarbon solvent recovery method for recovering oil from subsurface formations that are penetrated by at least one injection well, one production well, and one interior production well which extend from the surface of the earth and into the subsurface formation containing viscous hydrocarbons.
2. Description of the Prior Art
In copending application Ser. No. 285,696, filed July 22, 1981, to John L. Fitch, et al., there is disclosed a method of recovering viscous oil from a viscous oil-bearing subsurface formation wherein a predetermined amount of solvent is injected into a fluid communication path formed in the bottom of the formation intermediate an injection well and a production well. Thereafter, the formation is allowed to undergo a soak period for a time sufficient for gravity-driven convection to substantially mix the solvent with the oil in the formation thereby reducing its viscosity. The less viscous formation oil is then recovered by a fluid drive process utilizing a gas or liquid as the driving fluid.
In copending application Ser. No. 46,275, now U.S. Pat. No. 4,293,034, filed June 7, 1979, to John L. Fitch, there is disclosed a method of recovering viscous oil from a viscous oil-bearing subsurface formation wherein a solvent is injected into a high mobility channel formed in the bottom of the formation intermediate an injection well and a production well. The solvent is injected until the ratio of produced oil to solvent becomes unfavorable and thereafter the injection of solvent is terminated and gas is injected into the high mobility channel to produce solvent and oil from the formation. In U.S. Pat. No. 3,838,738 there is described a method for recovering viscous petroleum from petroleum-containing formations by first establishing a fluid communication path low in the formation. A heated fluid is then injected into the fluid communication path followed by injecting a volatile solvent such as carbon disulfide, benzene, or toluene into the preheated flow path and continuing injecting the heating fluid and recovering fluids including petroleum from the production well.
In U.S. Pat. No. 3,500,917 there is disclosed a method for recovering crude oil from an oil-bearing formation having a water-saturated zone underlying the oil-saturated zone. A mixture of an aqueous fluid which has a density greater than the density of the crude oil and a solvent having a density less than the density of the crude oil are injected into the water-saturated zone and oil is produced from the formation.
U.S. Pat. No. 4,026,358 discloses a method for recovering heavy oil from a subterranean hydrocarbon-bearing formation traversed by at least one injection well and one production well wherein a slug of hydrocarbon solvent in amounts of 0.1 to about 20 percent of the formation pore volume and having a gas dissolved therein is injected into the formation via the injection well. Thereafter, a thermal sink is created in the formation by in-situ combustion or by injecting steam. The wells are then shut-in for a predetermined time to permit the formation to undergo a soak period, after which production is continued. Optionally, after the production period, the formation may be water-flooded to recover additional oil from the formation.